(MECE 337) DESIGN OF MECHANICAL COMPONENTS Summer 2023 Kai Jongho Kwon, PhD, PE Associate Professor of Engineering Practice Department of Mechanical Engineering CHAPTER 1 Introduction to Mechanical Engineering Design References: • Engineering Design (6th Ed.), Dieter and Schmidt (McGraw Hill) • Engineering Mechanics: Statics, Dynamics (14th Ed.), Hibbeler (Pearson) • Mechanical Engineering Reference Manual (13th Ed.), Lindeburg (PPI) • Images on Google (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) CONTENTS Chapter 1 2 Introduction to Mechanical Engineering Design Introduction 3 1-1 Design 4 1-2 Mechanical Engineering Design 5 1-3 Phases and Interactions of the Design Process 6 1-6 Standards and Codes 8 1-9 Stress and Strength 10 1-11 Design Factor and Factor of Safety 12 1-14 Dimensions and Tolerances 13 1-15 Units 23 1-16 Calculations and Significant Figures 36 Problems and Exercises (MECE 337) Design of Mechanical Components (Chapter 1) 37 Lecture #01 (Dr. Kwon) Introduction 3 Chapter 1 – OVERVIEW • Mechanical design is a complex process, requiring many skills. • Extensive relationships need to be subdivided into a series of simple tasks. • The complexity of the process requires a sequence in which ideas are introduced and iterated. Chapter 1 – OBJECTIVE • This chapter explains some differences between design and analysis. • Also, it introduces some fundamental notations and approaches to design. Chapter 1 – PRACTICE • Solve and practice the problems below in the textbook for quizzes and exams. • Homework problems should be selected from the list below. 1-12 1-14 1-23 (MECE 337) Design of Mechanical Components (Chapter 1) 1-32 Lecture #01 (Dr. Kwon) 1-1 Design 4 Design and Product • Design is either to formulate a plan for the satisfaction of a specified need or to solve a specific problem. • Design process requires innovation, iteration, and decision-making. • Creativeness, communication-intensive ability, and problem-solving skill are needed for a designer. • Products should be: Functional, Safe, Reliable, Competitive, Usable, Manufacturable, and Marketable. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-2 Mechanical Engineering Design 5 Mechanical Engineering Design and Disciplines • Mechanical engineers are associated with the production and processing of energy and with providing the means of production, the tools of transportation, and the techniques of automation. • Mechanical engineering design involves all the disciplines of mechanical engineering, such as mechanics of solids and fluids, materials and manufacturing processes, thermomechanical treatments, statistical descriptions, and electrical and information theory, etc. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-3 Phases and Interactions of the Design Process 1 6 Design Process • The design process is iterative in nature, and requires initial estimation, followed by continued refinement, and ended with the presentation of the planes for satisfying the need. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-3 Phases and Interactions of the Design Process 2 7 Design Considerations • Various characteristics must be considered and provided in a design: 1 2 3 4 5 6 7 8 9 10 11 12 13 Functionality Strength/stress Distortion/deflection/stiffness Wear Corrosion Safety Reliability Manufacturability Utility Cost Friction Weight Life 14 15 16 17 18 19 20 21 22 23 24 25 26 (MECE 337) Design of Mechanical Components (Chapter 1) Noise Styling Shape Size Control Thermal properties Surface Lubrication Marketability Maintenance Volume Liability Remanufacturing/resource recovery Lecture #01 (Dr. Kwon) 1-6 Standards and Codes 8 1 Standards • Sets of specifications for parts, materials, or processes. • Intended to achieve uniformity, efficiency, and a specified quality. • Limits the multitude of variations. Codes • Sets of specifications for analysis, design, manufacture, and construction of something. • To achieve a specified degree of safety, efficiency, and performance or quality. • Does not imply absolute safety. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-6 Standards and Codes 9 2 Standards and Codes in Mechanical Engineering • Various organizations establish and publish standards and codes of particular interest to mechanical engineers for common and/or critical industries. Aluminum Association (AA) American Bearing Manufacturers Association (ABMA) American Gear Manufacturers Association (AGMA) American Institute of Steel Construction (AISC) American Iron and Steel Institute (AISI) American National Standards Institute (ANSI) American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) American Society of Mechanical Engineers (ASME) American Society of Testing and Materials (ASTM) American Welding Society (AWS) ASM International British Standards Institution (BSI) Industrial Fasteners Institute (IFI) Institute of Transportation Engineers (ITE) Institution of Mechanical Engineers (IMechE) International Bureau of Weights and Measures (BIPM) International Federation of Robotics (IFR) International Standards Organization (ISO) National Association of Power Engineers (NAPE) National Institute for Standards and Technology (NIST) Society of Automotive Engineers (SAE) (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-9 Stress and Strength 1 10 Strength • An inherent property of a material or of a mechanical element. • Depends on treatment and processing. • May or may not be uniform throughout the part. • Examples: ultimate strength, yield strength. Stress • A state property at a specific point within a body. • Primarily a function of load and geometry. • Sometimes also a function of temperature and processing. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-9 Stress and Strength 2 11 Engineering Stress–Strain Curve • Ferrous metals: Low carbon steels (Mild steels) (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-11 Design Factor and Factor of Safety 12 Design Factor Method • Focus on the survival probability of the function and reliability of a design. • Since stress may not vary linearly with load, it is more common to express the design factor in terms of strength and stress. nd loss-of-function strength S allowable stress (or ) • Stress and strength terms must be of the same type and units. • Stress and strength must apply to the same critical location in the part. • The factor of safety is the realized design factor of the final design, including rounding up to standard size or available components. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances 13 1 Dimensions and Tolerances • Engineering drawings include information to specify dimensions and features for manufacturing a profitable product: - Size of feature. - Relative position between features. - Required precision (tolerance) of sizing and positioning features. - Type of material and instructions on processing. • Tolerances: - Acceptable variance in a dimension. - Tolerances limit the permissible variations in size because it is impossible to repeatedly manufacture a part exactly to a given dimension. - Small (tight) tolerances require more costly processes. - Bilateral – Variation in both directions (e.g., - Unilateral – Variation in only one direction (e.g., (MECE 337) Design of Mechanical Components (Chapter 1) or . . or . . ). . . ). Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances Dimensions 14 2 1 • Dimensions indicate size, location, and orientation of all part features. Engineering drawing: Top view Front view Size (MECE 337) Design of Mechanical Components (Chapter 1) (Right side view) Location & Orientation Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances Dimensions 3 15 2 • Section view to clarify dimensioning of internal features. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances 4 16 Standard Sizes • Cost is almost always an important factor in engineering design. • Use of standard sizes is a first principle of cost reduction. • Table A–17 lists some typical preferred sizes. • Certain common components may be less expensive in stocked sizes. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances Clearance and Interference 17 5 1 • Clearance is the distance between the outer diameter of the shaft and the inner diameter of the bearing. • The clearance is used only when the internal member (shaft) is smaller than the external member (bearing). • The diametral clearance and radial clearance are used in dimensioning. • Interference is the opposite of clearance (negative clearance), used only when the internal member (shaft) is larger than the external member (bearing). (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances Clearance and Interference 18 6 2 • Example: Maximum clearance, (Clearance) Minimum clearance, (Interference) (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances 19 7 Types of Fits • Fit describes the nature of the space between two mating parts, particularly with a hole (bearing) and shaft. • Clearance fit – When the hole is at the maximum tolerance and the shaft is at its minimum tolerance, there is some space (zero or positive clearance) between the two parts, and it allows free rotation or sliding. • Interference fit – When the shaft diameter is larger that the hole diameter, a press fit with negative clearance is needed to assemble the two pieces by heating the outer body and/or cooling the inner body. • Transition fit – When the maximum clearance is positive and the minimum clearance is negative, either slight clearance or slight interference is possible to provide accurate location. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances Tolerance Stack-Up 20 8 1 • When two or more parts are assembled in contact, the tolerance of the assembly is affected by the cumulative effects of multiple tolerances. • Example #1: If all dimensions are at the top of the tolerance limit: 𝐿 = 18.01 + 30.01 + 18.01 + 20.01 = 86.04 mm If all dimensions are at the bottom of the tolerance limit: A Datum C B D L Tolerance on each dimension 𝐴 = 18 ± 0.01 mm 𝐵 = 30 ± 0.01 mm 𝐶 = 18 ± 0.01 mm 𝐷 = 20 ± 0.01 mm 𝐿 = 17.99 + 29.99 + 17.99 + 19.99 = 85.96 mm The tolerance stack-up on the overall length: 𝑇 =𝐿 −𝐿 = 86.04 − 85.96 = 0.08 mm 𝑇 = Σ𝑇 = 0.02 + 0.02 + 0.02 + 0.02 = 0.08 mm The overall length: 𝐿=𝐿 ± 𝑇 /2 = 86 ± 0.04 mm (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances Tolerance Stack-Up 21 9 2 • When two or more parts are assembled in contact, the tolerance of the assembly is affected by the cumulative effects of multiple tolerances. • Example #2: The fixed tolerance of the overall length 𝐿 is required. The limits on the dimension at the right end 𝐷: A Datum C B D unspecified L Tolerance on each dimension 𝐴 = 18 ± 0.01 mm 𝐵 = 30 ± 0.01 mm 𝐶 = 18 ± 0.01 mm 𝐿 = 86 ± 0.01 mm 𝐷 = 86.01 − 17.99 − 29.99 − 17.99 = 20.04 mm 𝐷 = 85.99 − 18.01 − 30.01 − 18.01 = 19.96 mm 𝑇 =𝐷 𝐷=𝐷 −𝐷 = 20.04 − 19.96 = 0.08 mm ± 𝑇 /2 = 20.00 ± 0.04 mm The tolerance on 𝐷 is four times the tolerance on the other hole locations due to the tolerance stack-up. However, the overall part dimensions are controllable by the tolerance on 𝐷. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-14 Dimensions and Tolerances 22 10 Elimination of Redundant Dimension • When the overall part dimensions are given, it is not necessary to give the last position dimension. • With all the four dimensions given, the part is over-constrained because of tolerance stack-up. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 1 23 Newton’s Law of Motion 1st law: The Law of Inertia • A particle, originally at rest or moving in a straight line with constant velocity, tends to maintain the equilibrium state. 2nd law: The Law of Acceleration • A particle acted upon by an unbalanced force F experiences an acceleration a. • The acceleration has the same direction as the force and a magnitude that is directly proportional to the force. 3rd law: The Law of Action and Reaction • The mutual forces of action and reaction between two particles are equal, opposite, and collinear. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 24 2 Newton’s Second Law of Motion • Newton’s first and third laws were used in developing the concepts of Statics. • Newton’s second law forms the basis of the study of Dynamics. • Newton’s second law of motion can be expressed in a mathematical form. • The vector is the resultant unbalanced force acting on a particle. A particle acted upon by an unbalanced force experiences an acceleration . • The vector is the acceleration of the particle. The acceleration has the same direction as the force and a magnitude that is directly proportional to the force. • The positive scalar is the mass of the particle. • Newton’s second law cannot be used when the particle’s speed approaches the speed of light, or if the size of the particle is extremely small such as an atom. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 25 3 Newton’s Law of Gravitational Attraction • Any two particles or bodies have a mutually attractive gravitational force acting between them. G = universal constant of gravitation G = 66.73(10-12) m3/(kg∙s2) m1, m2 = mass of each of the two particles r = distance between the two particles Weight • When near the surface of the earth, the only gravitational force having any sizable magnitude is that between the earth and the body. • This force is called the weight of the body. g = gravitational acceleration g = 9.807 m/s2 = 9.81 m/s2 g = 32.174 ft/s2 = 32.2 ft/s2 (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 26 4 Mass and Weight • It is obviously important to understand the difference between the mass and weight of a body. • The mass is an absolute property of a body. It is independent of the gravitational field in which it is measured. • The mass provides a measure of the resistance of a body to a change in velocity, as defined by Newton’s second law of motion ( ). • The weight of a body is not absolute, since it depends on the gravitational field in which it is measured. • Weight is defined as , where is the acceleration due to gravity. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 27 5 Systems of Units 1 • Any physical quantity can be characterized by dimensions. • The magnitudes assigned to the dimensions are called units. • Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary or fundamental dimensions, while others such as velocity V, energy E, and volume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions. • Metric SI system: A simple and logical system based on a decimal relationship between the various units. • Metric systems are usually absolute systems. • English (or USCS) system: It has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily. • English-based systems are usually gravitational systems. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 28 6 Systems of Units 2 • Four basic quantities (length, time, mass, and force) are related by Newton’s second law of motion, = . • Gravitational acceleration (g): (SI) g = 9.807 m/s2 = 9.81 m/s2 (USCS) g = 32.174 ft/sec2 = 32.2 ft/sec2 (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 29 7 SI Unit System • The International System of units • Metric system of units: MKS (Meter-Kilogram-Second) • Force: - [N] = [kg∙m/s2] N = newton N (newton) is equal to a force required to give an acceleration of m/s2. kg (kilogram) of mass - Force has the derived unit in the SI units. • Weight: - A body of mass [N] = [kg∙m/s2] kg has a weight of g= m/s2 N. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 30 8 USCS Unit System 1 • The United States Customary System of units • English system of units: FPS (Foot-Pound-Second) • Force: - [lb] = [slug][ft/sec2] [slug] = [lb∙sec2/ft] slug is equal to the amount of mass accelerated at by a force of lb. ft/s2 when acted upon - Mass has the derived unit in the USCS units. • Weight: - A body of mass [lb] = [slug][ft/sec2] slug has a weight of g = 32.2 ft/sec2 lb. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 31 9 USCS Unit System 2 • The derived unit of mass [slug] is not often used in actual problems. • The unit of mass [lbm] is commonly used in engineering problems. • The unit of force [lbf] is also used to avoid confusion with the units between mass and force. • However, Newton’s second law has an inconsistent system of units when [lbf], [lbm], and [ft/sec2] are applied. • Therefore, the force unit conversion factor must be included: lbm-ft/(lbf-sec2) • Force: [lbf] = [lbm] = [ft/sec2] • Weight: [lbf] = [lbm] = 32.2 ft/sec2 (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 32 10 USCS Unit System 3 • Calculate the weight in lbf of a 2.00 lbm object in a gravitational field on the earth. • The weight in the unit of pound force [lbf] of an object has the same numerical quantity as that of mass in the unit of pound mass [lbm] on the earth. In this case, the force unit conversion factor must be included to get dimensionally homogeneous values on the both sides of the equation. • However, the unit of pound [lb] is commonly used for just weight in engineering problems. In that case, the mass must be converted with the unit of slug. • Consequently, the above two cases are based on the same concept. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 11 33 Conversion of Units • 1 in = 0.0254 m = 2.54 cm = 25.4 mm • 1 ft = 12 in (inches) = 0.3048 m = 30.48 cm = 304.8 mm • 1 mi (mile) = 5280 ft = 1609.34 m = 1.61 km • 1000 lb = 1 kip (kilo-pound) • 2000 lb = 1 ton (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 34 12 Prefixes of SI Units • When a numerical quantity is either very large or very small, the unit used to define its size may be modified by using a prefix. • The kilogram (kg) is the only base unit that is defined with a prefix. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-15 Units 35 13 Rules of SI Units • No plurals - m = 5 kg, not kgs • Separate units with a dot ( ∙ ) - meter-second = m∙s ; ms = milli-second ; N = kg∙m/s2 = kg∙m∙s-2 • Most symbols are in lowercase. - Key exceptions are N, Pa, M and G. • Exponential powers apply to both the units and their prefixes. - cm∙cm = (cm)2 = cm2 ; mm∙mm = (mm)2 = mm2 ; μm∙μm = (μm)2 = μm2 • Do not use a prefix in the denominator of composite units. - N/mm → kN/m ; m/mg → Mm/kg (kg is the base unit.) • The final result should be expressed using a single prefix and the numerical values between 0.1 and 1000. - (50 kN)(60nm) = [50(103) N][60(10-9) m] = 3000(10-6) N∙m = 3(10-3) N∙m = 3 mN∙m (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) 1-16 Calculations and Significant Figures 36 Dimensional Homogeneity • The term of any equation must be dimensionally homogeneous. • Each term of both sides must be expressed in the same units. Significant Figures and Rounding Off Numbers • Three significant figures are generally used in engineering applications. 3.5587 → 3.56 0.5896 → 0.590 9.3866 → 9.39 1.341 → 1.34 0.3762 → 0.376 9.871 → 9.87 72.25 → 72.3 0.1275 → 0.128 0.2555 → 0.256 23400 → 23.4(103) 0.00821 → 8.21(10-3) 0.000582 → 0.582(10-3) = 582(10-6) • Most of the original data used in engineering calculations do not have accuracy better than one percent. Calculations • Do not round off calculations until expressing the final results. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) Problems and Exercises 37 1 1. Find the missing dimension AB and its bilateral balanced tolerance. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) Problems and Exercises 38 2 2. Start with point B and go clockwise around the circuit to find the maximum and minimum gaps of AB at the wall based on its dimension and tolerance. (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) Problems and Exercises 39 3 2. (continued) • The gap and tolerance can be determined by completing the table below. Dimension Section Positive Tolerance Negative Across snap ring Across sleeve Across washer Washer to wall Totals ④ ----Positive total Negative total Basic gap ----① ----- ----- ----- ----- ② Max. gap ⑤=③+④ ③=|①-②| Min. gap ⑥=③-④ (MECE 337) Design of Mechanical Components (Chapter 1) Lecture #01 (Dr. Kwon) Problems and Exercises 40 4 3. What is the fracture toughness in units of toughness of 35 ? (MECE 337) Design of Mechanical Components (Chapter 1) when a material has a fracture Lecture #01 (Dr. Kwon)
